Steam and gas sterilization methods using either steam or ethylene oxide (or other disinfecting gases) are used for sterilizing a wide range of medical products from pharmaceutical preparations to surgical instruments.
A sterilizing method must effectively kill all organisms, including bacterial spores, without damage to the article or goods being sterilized. However, many sterilizing gases which meet these criteria, such as ethylene oxide, have been recognized to expose workers and the environment to safety hazards. Legislation is severely restricting the amount of hazardous gases such as ethylene oxide (a carcinogen) in the working environment, or the use of any system or method which produces toxic residues or exhaust products. This is presenting a major crisis in hospitals and other areas of the health industry. Although steam avoids this hazard, it is not useful for items and packaging materials that are unable to withstand high temperatures.
Sterilizing methods can be monitored by components such as biological indicators (BIs) and chemical indicators, both of which are used to monitor different performance aspects of sterilizing methods.
The U.S. Pharmacopeia XXII, Official Monograph, pp. 1625-1626 defines a biological indicator as "a characterized preparation of specific microorganisms resistant to a particular sterilization process. It is used to assist in the qualification of the physical operation of sterilization apparatus in the development and establishment of a validated sterilization process for a particular article, and the sterilization of equipment, materials, and packaging components for aseptic processing. It may also be used to monitor a sterilization cycle, once established, and periodically in the program to revalidate previously established and documented sterilization cycles. It is in one of two main forms, each of which incorporates a viable culture of a known species of microorganism. In one the spores are added to a carrier (disk or strip of filter paper, glass, or plastic) and packaged so as to maintain the integrity of the inoculated carrier but, when used appropriately in the individual immediate package, so as to allow the sterilizing agent to exert its effect. In the other, the spores are added to representative units of the lot to be sterilized (inoculated product) or to similar units (inoculated similar product)."
The need to develop sterilization methods avoiding the use of a hazardous gas such as ethylene oxide and the desirability of sterilization processes suitable for packages containing cellulose or those whose integrity can be destroyed at higher pressures or temperatures has lead to other sterilization processes, and other biological indicators. For example, Bacillus stearothermophilus, which is recognized as an organism useful in biological indicators for steam sterilizations, has been found to have a curvilinear response curve to oxidizing gas sterilization processes. This is disadvantageous because inactivation of an indicator organism must occur in a predictable manner.
Chemical indicators are generally used to monitor whether or not an article has been exposed to sterilizing conditions. A chemical indicator response is not necessarily an indication of sterility because it only indicates that the chemical indicator and any accompanying articles has been processed in a sterilizer.
Chemical indicators have been developed and are used with both of the prevalent sterilization processes: steam and ethylene oxide. For example, U.S. Pat. No. 4,914,034, issued Apr. 3, 1990, inventors Welsh and Dyke, describes disposable test packs for monitoring steam and ethylene oxide sterilization cycles, which include a chemical process indicator strip. U.S. Pat. No. 4,671,936, issued Jun. 9, 1987, inventor Barron, describes a cation exchange resin for monitoring alkylene oxide (including ethylene oxide) cycles.
Plasma gas sterilizer systems are described in U.S. Pat. Nos. 3,851,436 and 3,948,601 and comprise separate plasma RF generation chambers and sterilizing chambers. A gas plasma is produced in the plasma generating chamber with argon, helium, nitrogen, oxygen, or xenon, which is passed into a separate sterilization vacuum chamber containing the articles to be sterilized.
Numerous other gas plasma sterilizers using a wide variety of gases have been described in the literature. A few have been commercially produced. For example, one system is described in U.S. Pat. No. 4,643,876. In this system, the articles to be sterilized are pretreated with vapor from a solution of hydrogen peroxide and subjected to an electromagnetic field. The plasma is formed from the interaction of the electromagnetic field and the water and hydrogen peroxide vapors. Hydrogen peroxide is ultimately converted to water and oxygen in this process, thus eliminating toxic hydrogen peroxide residuals. The '876 patent purposely adds peroxide to the sterilizing chamber. Another similar patent, U.S. Pat. No. 4,756,882, discusses a system in which the source of the hydrogen peroxide is residue left on instruments.
U.S. Pat. No. 5,244,629, inventors Caputo et al., issued Sep. 19, 1993, describes a process for plasma sterilization including exposing an article in a sterilizing chamber to at least one combination sterilizing cycle. Each combination sterilizing cycle includes a pulsed treatment with gaseous antimicrobial agent, removal of the gaseous antimicrobial agent, and a plasma treatment. The pulsed treatment includes one or more pulse-vacuum cycles, each pulse-vacuum cycle includes the steps of evacuating the sterilizing chamber and exposing the article to the gaseous antimicrobial agent with a predetermined pressure profile during a predetermined time. The gaseous antimicrobial agent is preferably carried in a gas mixture with a nonreactive carrier gas. In one embodiment, the predetermined pressure is pulsed. In another embodiment, it is ramped. After the pulsed treatment, the antimicrobial agent is removed by evacuating the sterilizing chamber. The plasma treatment includes exposing the article to a plasma having essentially uncharged, highly reactive free radicals and atoms.
BIER vessels are known and used to evaluate the resistance performance of biological indicators that are intended for use in monitoring either ethylene oxide or steam sterilization cycles. A description of standards for BIER/EO gas vessels is set out by the Association for the Advancement of Medical Instrumentation (AAMI BEOV--3/82). Similarly, the Association describes performance requirements for the equipment used to determine resistance performance patterns for biological indicators exposed to saturated steam at various temperatures (AAMI BSV--3/81).
However, the performance of a biological indicator intended for use in sterilizers utilizing plasmas has not been adequately monitored by the prior art BIER vessels, and the sterilization conditions in plasma sterilizers are different from those in EO and steam sterilizers. Also, particular strains of microbial spores selected for use as a biological indicator for one type of sterilization process are often not suitable for other sterilization processes or even for differing sterilizing conditions of the same mode of sterilization. The rate at which spores of a biological indicator are inactivated depends on sterilant concentration, so it is important to have a means to determine the concentration of sterilant in a BIER vessel. Further, BIER vessels are, in a limited sense, a type of sterilizer, but BIER vessels must permit significantly greater operator control and are also used to perform "partial" sterilizing cycles in order to obtain survivor curves for biological indicators.
Accordingly, one object of the present invention is to provide a vessel suitable for testing the performance of indicators (biological and/or chemical) when monitoring plasma sterilization processes.